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difference between beta and gm

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Member level 5
Jun 10, 2010
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When would one consider beta and when would one consider gm for design using BJT? Thanks.

I think you are mixing BJT gain beta with FET transconductance gm.
Beta is the ratio of collecter current to base current i.e input is base current Ib and output is collector current Ic.
Transconductance is the ratio of drain current to gate voltage i.e input is gate to source voltage Vgs and output is drain current Id.

thats not true. in bjt rpi=beta/gm. what i am wondering is generally beta varies alot over the process and i have read in the books that you should make beta insensitive design, so wondering does gm have any different role in design?
β=ΔIc/ΔIb for AC signal
β=Ic/Ib for DC for bias point

That's is current gain

however gm is the transconductance parameter

gm = Ic/VΠ for the hybrid model pi
thats correct Miguel, but what is the practical significance of it in the design?
When designing using bjts, I think β should be considered more of a parameter the transistor came with from the fabrication process, e.g. β=100 for a npn or β=20 for a pnp. The collector current is the base current multiplied by this factor β.
On the other hand, gm is more of a parameter that user can adjust depending on specs. At room temp. the gm can be approximated by 40Ic, where Ic is the PSF current (no ac biassing). So depending what Q point you chose for bjt, you will have a certain gm.
From a practical standpoint, if you are designing with discrete BJT transistors (as opposed to IC design), Beta is given in transistor data sheets, gm is not.

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